As you can tell by the title, this is for discussion about the Dark Side transitions in the KotOR games. Many people have argued over what they should really be like. I myself have pondered this and I have come to this conclusion:

The Dark Side transitions of the KotOR era reflect the times and the state of the Galaxy. Think about it.
The Mandalorian Wars caused the death of thousands of innocents and the extinction of entire races. And Malak and Revan's war on the Galaxy only increased the death toll. Perhaps these deaths and wounds sent echos through the force, echos that presented themselves on those that walked the path of the Dark Side and opened themselves up to it. This could attribute to the diseased look of Dark-siders in K1.

In K2, when the Galaxy is still healing from the aforementioned wars, entire planets of Force sensitives are being devoured and left void. Jedi are being murdered across the Galaxy and the Dark Side reigns supreme. Imagine the echos that these events could have caused. Not only could the echos in the force manifest on those who walked the Dark Path, but the pure corruption of the Dark Side could also show itself. As seen in the rotten zombie like transitions on K2.

During the Clone Wars and the Galactic Civil War, there weren't nearly as many deaths, and the Force was pretty evened out (due to the presence of Jedi and Sith in both eras), which is probably why Palpatine/Sidious was able to hide his presence for so long.